taru-adjective, adverb taking the 'to' particle
trembling with fear; filled with trepidation
Archaic yojijukugo describing a state of extreme fear or anxiety, often used in literary or historical contexts. Functions as a taru-adjective or adverb with と.
彼は戦々慄々として敵の到来を待った。
He waited for the enemy's arrival, trembling with fear.
戦々慄々たる思いで試験の結果を見た。
I looked at the exam results with a feeling of trepidation.
A yojijukugo composed of 戦 (war, battle, tremble) and 慄 (fear, tremble), each repeated for emphasis. The exact historical derivation is uncertain, but it is a classical Chinese-derived expression for extreme fear.